1. Field of the Invention
This invention concerns a control device for the turbine bypass valve of a turbocharged internal combustion engine which is supercharged by at least one turbo-compressor driven in a rotary manner by the residual energy of the exhaust gases.
2. Description of the Prior Art
The technique for the air supercharging of internal combustion engines by using the residual energy of the exhaust gases has developed over many years in various and gradual ways. It was initially used for aviation motors with controlled spark so as to regain their ground-level power at high altitudes, where the density of the air taken in is low, without damaging the engine which, under such circumstances, is suitably cooled by the relatively cold outside air. Following the Second World War, developments were oriented toward increasing the power and output of large diesel motors for which the exhaust energy, less in this type of motor, nevertheless made is possible to install centrifugal turbocharging units which were economically advantageous due to the scale effect of the large size of such motors used as marine engines, high-power fixed groups and, finally, railroad locomotive engines. With the technological progress made with turbocharged engines, in particular as regards the turbine rotor subjected to high exhaust temperatures (ranging from 600.degree. C. to 800.degree. C.), it became economically possible to install rotary or pressure-wave supercharging on the diesel engines of heavy trucks, and subsequently of luxury automobiles. A further economic and technological shift relating to turboblower supercharging units, prompted by the sizable jump in the cost of energy, now makes it possible to install supercharging devices on internal combustion and controlled-spark engines so as to reduce their displacement with loss of power.
However, the use of air supercharging by means of tapping the residual energy of the exhaust gases creates new technical problems when applied to controlled-spark motors. As it happens, the supercharging pressure for this type of engine tends to be at its peak at low revolutions per minute and under heavy engine load, which inevitably results in the appearance of a preignition and knocking phenomenon which is harmful under these operating conditions and stems not from too high a compression ratio, but rather from excess pressure of the carbureted mixture prior to compression. In order to overcome this difficulty, a control valve bypassing the turbine is installed in such a way that the exhaust flap of said valve is connected to a piston which is subjected, in the opening direction and in opposition to flap closing spring, to the feed pressure upstream from the compressor and downstream from the carburetor or fuel injection device. In this manner, when the air supercharging pressure of the motor exceeds a value predetermined by the calibration of the flap closing spring, the flap gradually closes and lowers the revolutions per minute of the turbine so as to stabilize the supercharging pressure at the predetermined value.
An examination of the performance obtained with vehicles currently equipped with small or medium displacement engines provided with turboblower supercharging devices reveals that, at the moderate speeds observed by the majority of the users of such vehicles, operators did not obtain the reductions in fuel consumption they were entitled to expect on the basis of the results obtained at high speeds, and that these mediocre results at moderate speeds were attributable in part to the exhaust back pressure imparted by the placement of the turbine rotor in the exhaust gas circuit.